Optical memory element and manufacturing method thereof

ABSTRACT

A method for manufacturing an optical memory element including the steps of disposing a resist film on a substrate, placing a photo-mask carrying a guide groove pattern on the resist film so as to position the photo-mask over a predetermined position of the substrate, forming a guide groove pattern latent image on the resist film, developing the guide groove pattern latent image formed on the resist film, conducting an etching operation through the developed guide groove pattern as to form guide grooves in the substrate, removing the resist film from the substrate, and disposing a recording medium layer on the substrate having the guide grooves formed therein.

This application is a continuation of application Ser. No. 06/901,132filed on Aug. 28, 1986, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to an optical memory element capable forat least one of the recording, reproducing and erasing operations bymeans of optical beam irradiation, or more specifically to an opticalmemory element substrate and a photo-mask for use in transcribing guidetracks and track address patterns onto an optical memory elementsubstrate.

Of late, demand for optical memory elements is increasing year by yeardue to their high density and large capacity. Optical memory elementscan be classified into the three types in terms of their use: read-onlymemory, add-on memory and erasable memory.

Optical memory elements ensure high density and large capacity because abit (information recording unit), which is determined solely by opticalbeam diameter, can be made as small as about 1μm in size. This fact,however, causes a number of restrictions for an optical memory system;optical beam positioning must be extremely accurate in order to recordinformation precisely in a predetermined point or reproduce properinformation recorded in a preselected point. In the case of read-onlytype optical memory, in general, address data can be recorded togetherwith data information so that it is possible to properly position theoptical beam while recorded data information is being reproduced. Foradd-on memory or erasable memory type, on the other hand, it isdifficult to record address data together with data information in thememory. Therefore, in the add-on memory or the erasable memory, guidesignals or guide addresses are normally recorded on the memorysubstrate.

For instance, an optical memory element to be used as add-on or erasablememory normally has guide tracks in the substrate, to guide aninformation-recording or information-reproducing beam spot to aspecified position on the optical memory element. In many cases, trackaddress-indicating data is written in a part of each guide track tolocate the guide track.

FIG. 1 shows the essential part of the memory substrate of the add-on orerasable memory type in perspective view. As shown, stripe-shapedgrooves are formed in the substrate, and information is recorded orreproduced along the grooves. Though not shown, the grooves areintermittent in the circumferential direction so as to provide addressbit information for each groove. In an optical memory element of a discshape as shown in FIG. 2, in particular, guide track 3 and tracksaddress portion 2 (together with sector addresses if the tracks aredivided into many sectors) are formed concentrically or spirally in thesubstrate 1. For the purpose of simplicity, only one track 3 and onlyone address portion 2 are shown in FIG. 2.

A number of methods have been practiced for manufacturing a discsubstrate provided with tracks and addresses as described above. A maskwhich carries guide tracks and addresses thereon may be airtightlyplaced on a resist film applied on a glass disc, to form the tracks andaddresses in form of grooves or pits directly on the glass disc. In thismethod, it is essential to form the guide tracks in such a manner thatthe guide track center coincides with the center hole of the glass discas precisely as possible. With poor concentricity between the guidetracks and the glass disc, when the glass disc is rotated for recording,reproduction or erasing with the center hole fixed on a rotary shaft 4,as shown in FIG. 3, the guide tracks vibrate significantly with respectto a recording, reproducing or erasing laser beam 5, and hampering trackservo operation for controlling the lens position. For the above reason,it is necessary to join the guide track center and the glass disc centerhole as precisely as possible (with deviation allowance preferablywithin 20μm) in the optical exposure of the guide tracks.

The above manufacturing method can cause another problem in the guidetrack and track address pattern transcription process. The resist film6, when applied, tends to have a partial rise 8 at the peripheralportion of the disc glass as shown in FIG. 4. This rise 8 often impairsclose contact between the resist film 6 and the photo-mask 7, althoughit is desirable that they are made in close contact with each other. Thewidth of this deficient close contact is usually 1-2mm. In FIG. 4, theguide track and address pattern on the photo mask 7 is omitted. Thedeficiency in close contact prevents the guide tracks and trackaddresses from being formed at proper positions.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present invention toprovide an optical memory element in which guide tracks and trackaddresses are accurately formed on the substrate of an optical memoryelement.

Another object of the invention is to provide a method for manufacturingan optical memory element, which method ensures accurate formation ofguide tracks and track addresses.

Another object of the invention is to provide a mask which facilitatesconcentricity between the guide track center and the disc center holewhen transcribing guide tracks and addresses on a resist film applied onthe disc.

Still another object of the present invention is to provide a photo-maskwhich prevents deficient close contact between a resist film and thephoto mask from being caused by a rise of the resist film on theperipheral portion of the disc glass.

Further objects of the present invention are to provide improvements inthe configuration of glass substrates so as to achieve guide tracks andaddress-indicating grooves of more accurate shape.

Other objects and further scope of applicability of the presentinvention will become apparent from the detailed description givenhereinafter. It should be understood, however, that the detaileddescription and specific examples, while indicating preferredembodiments of the invention, are given by way of illustration only,since various changes and modifications within the spirit and scope ofthe invention will become apparent to those skilled in the art from thisdetailed description.

To achieve the above objects, according to an embodiment of the presentinvention, a method for manufacturing an optical memory element may beemployed which includes the steps of disposing a resist film on asubstrate, providing a photo-mask carrying a guide groove pattern on theresist film so as to position the photomask over a predeterminedposition of the substrate, forming a guide groove pattern latent imageon the resist film, developing the guide groove pattern latent imageformed on the resist film, conducting an etching operation through thedeveloped guide groove pattern so as to form guide grooves in thesubstrate, removing the resist film from the substrate, and disposing arecording medium layer on the substrate having the guide grooves formedtherein.

The substrate is disc shaped with a hole in its center. The photo-maskis provided with a marker of the shape corresponding to the center holein disc substrate and a pattern corresponding to guide tracks to beformed on the substrate disc.

The photo-mask may be cut away at a position corresponding to the riseof the resist film.

The peripheral portion of the substrate may be ground to be inclined sothat the plane of the substrate with the grooves formed therein ishigher than the rise of the resist film which may occur on theperipheral portion of the substrate when the resist film is applied onthe substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not limitativeof the present invention and wherein:

FIG. 1 is a perspective view of the essential part of a glass substrateof an add-on memory or erasable memory after grooves have been formedthereon;

FIG. 2 is a prespective view of a typical optical memory element(optical disc);

FIG. 3 is a sectional view showing a part of the optical disc system;

FIG. 4 is a sectional view showing the contact between the glasssubstrate and the photo-mask of a conventional optical memory element;

FIG. 5 is a sectional view for illustrating manufacturing steps of anoptical memory element of the present invention;

FIG. 6 is a plan view of an embodiment of an optical disc photo-mask ofthe present invention;

FIG. 7 is a plan view showing a part of another embodiment of an opticaldisc photo-mask of the present invention;

FIG. 8 is a partially enlarged sectional view of another embodiment ofan optical disc photo-mask of the present invention;

FIG. 9 is a sectional view showing the entire photo-mask of FIG. 8;

FIGS. 10 (a) and 10 (b) are partially enlarged sectional views showingother embodiments of an optical memory element photo-mask of the presentinvention;

FIG. 11 is also a partially enlarged sectional view showing stillanother embodiment of an optical memory element photo-mask of thepresent invention;

FIGS. 12 (a) and 12 (b) are plan views and a sectional view,respectively, of the glass substrate for an optical memory element;

FIGS. 13 (a), 13 (b) and 13 (c) are partially enlarged sectional viewsof glass substrates for an optical memory element of the presentinvention; and

FIGS. 14 (a), 14 (b) and 14 (c) are partially enlarged sectional viewsof the optical memory element glass substrates on which resist film isapplied.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of an optical memory element of the present invention willbe described in detail below with reference to the accompanyingdrawings.

FIG. 5 shows manufacturing steps of the substrate of the optical memoryelement of the present invention. Referring to FIG. 5, a manufacturingmethod of the optical memory element substrate is described by thefollowing steps.

Step I

An optical memory element glass substrate which is highly reliable withrespect to oxygen and moisture penetration (or which does not allowoxygen and moisture penetration) is cleaned. A photo resist film 6 isapplied on the glass substrate 1 (See FIG. 5 (a)). The thickness of thephoto resist film 6 is preferably about 100 nm-500 nm.

Step II

A mask plate 7 on which guide track and address information have beenpatterned is placed in an airtight manner over the photo resist film 6on the glass substrate 1. Then, the mask plate 7 is irradiated withultraviolet rays A to transcribe the mask pattern of the mask plate 7onto the photo resist film 6 (See FIG. 5 (b)). Since the optical memoryelement is disc shaped, it is desirable that the mask plate 7 is round.

Step III

The photo resist film 6 with the mask pattern thereon is developed so asto form grooves in the resist film 6 (See FIG. 5 (c)).

Step IV

The glass substrate 1 covered with the photo resist film 6 havinggrooves formed therein is subjected to wet etching operation or dryetching operation such as sputtering (reactive ion etching) in anetching gas atmosphere such as CF₄ or CHF₃. Grooves 8 are then formed inthe glass substrate 1 (See FIG. 5 (d)).

Step V

The developed resist film 6 is removed from the glass substrate 1 bymeans of sputtering in an O₂ atmosphere, or dissolving with a solventsuch as acetone. In this way, the glass substrate 1 with the grooves 8formed therein is produced (See FIG. 5 (e)).

Step VI

After the step V, a recording medium layer is formed on the glasssubstrate 1 having the grooves 8 formed therein.

The glass substrate 1 having grooves for guide tracks and guide addressinformation is completed through the above process. According to thisprocess, the mask plate 7 with the pattern for guide tracks and guideaddress information thereon, prepared in advance, is placed airtightlyover the glass substrate 1 coated with the photo resist film 6, so as totranscribe the mask pattern of the mask plate 7. Therefore, the timerequired for transcribing the guide pattern can be substantiallyreduced.

The optical disc mask 7 used in the above manufacturing process isdescribed as follow.

FIG. 6 is a plan view of an embodiment of an optical disc mask of thepresent invention. The mask 7 has been produced by forming Cr or CrOfilm over, for example, a quartz glass disc plate and etching the filmto make the pattern shown in FIG. 6. Referring to FIG. 6, the shadedpart indicates the area where Cr or CrO remains unremoved. The area "a"is an information area in a part of which are formed tracks 3 andaddresses 2 spirally or concentrically. Cr or CrO is removed in the area"b" of the mask 7. A positioning reference marker C is provided in thearea "b". The concentricity of the tracks can be secured by recordingthe reference marker C and the guide tracks simultaneously by using thesame recording device. For example, when the marker C has virtually thesame diameter as the glass disc center hole, the marker C can be easilyadjusted to the disc center hole when placing the mask on the glass discfor pattern transcription, so that it is easy to achieve theconcentricity of the tracks with respect to the disc.

The marker need not be limited to the above embodiment. As shown in FIG.7, two markers C may be recorded at an interval of about 10-20μm andwith such diameters that the glass disc center hole is positionedbetween the two markers. Assuming the glass disc center hole is about 15mm in diameter, for instance, the two markers may have the diameters ofabout 14.09 mm and 15.01 mm, respectively. With such markers, it is easyto position the mask with respect to the glass disc center hole.Generally, the center hole of a glass disc has a high degree ofroundness but often differs in the diameter, for instance within therange of about 15±0.5 mm, depending upon the manufacturing process. Toprovide for various center hole diameters of the disc, a plurality ofmarkers C with different diameters may be formed concentrically on amask so that the same mask can be used for discs with different centerhole diameters. Alternatively, instead of using the marker C of FIG. 6,it is possible to use the area "b" as a marker if the area "b" is madesmaller. In the above embodiment, Cr or CrO is used in the shaded areaof FIG. 6. Other metal films such as Ni, Ti or Ta may be used in placeof Cr or CrO.

As understood from the above, by using the mask of the presentinvention, it is possible to improve the concentricity of the memorydisc guide tracks with the memory disc center hole, thus minimizingvibration of the guide tracks with respect to the optical beam inrecording, reproducing or erasing information on the disc. Consequently,servo operation of an objective lens for condensing a laser beam becomeseasier.

Another embodiment of a mask 7 used in the manufacturing method of thepresent invention is described as follows.

FIG. 8 is a partially enlarged sectional view of another embodiment ofan optical memory element photo mask of the present invention. As shown,a groove 9 is formed in the photo-mask 7 at the position facing theperipheral portion of the glass disc 1, to accommodate the rise 8 of theresist film 6, thus eliminating deficient close contact between theglass disc 1 and the photomask 7.

For the glass disc of about 130±0.1 mm in diameter, for instance, thegroove desirably has the width "Y" (about 4 mm for example) with theinside diameter of a circle defined by the groove being "X" (127 mm forexample) as shown in FIG. 9. The depth "Z" of the groove should be about0.2-0.5 mm. The section of the groove is not necessarily square as shownin FIG. 9. It may be triangular as shown in FIG. 10 (a) or semi-circularas shown in FIG. 10 (b). In short, the groove provided in the photo maskmay be of any shape as long as it can absorb the rise 8 of the resistfilm 6 on the periphery of the glass disc 1. In this sense, instead offorming a groove, the peripheral portion 10 of the photo-mask 7 may bemade thinner than the central portion as shown in FIG. 11, to absorb therise 8 of the resist film 6.

By using the photo-mask of the present invention, the guide track andtrack address pattern for an optical memory element can be properlytranscribed onto the resist film applied on the optical memory elementsubstrate.

The shape of the section of the glass disc substrate is described withreference to the drawings as follows.

FIG. 12 (a) is a plan view of the glass disc substrate 1 and FIG. 12 (b)is a sectional view of the glass disc substrate 1. The outer edges 15 ofthe glass disc substrate 1 are chamfered, as shown.

FIG. 13 is an enlarged sectional view of the chamfered edge 15. FIG. 13(a) is a sectional view of the chamfered edge of a conventional glassdisc substrate, and FIGS. 13 (b) and 13 (c) are sectional views showingthe chamfered edge configuration of the glass disc substrate of thepresent invention. In both of FIGS. 13 (b) and 13 (c), the surfaceportion 17 without guide grooves is ground to be lower than the glassdisc substrate surface plane 18.

FIGS. 14 (a), 14 (b) and 14 (c) are sectional views of the glass discsubstrates having the edges shown in FIGS. 13 (a), 13 (b) and 13 (c),respectively, on which a resist film is applied by the spin coat method.At the edge of the glass disc substrate, the resist film has a rise 8due to the surface tension.

On the glass disc substrate of the conventional shape shown in FIG. 14(a), the rise 8 of the resist film 6 is higher than the resist filmsurface plane 11. When placing the mask plate 7 on the glass substrate 1in the manufacturing step II, under this condition, the mask plate 7will be positioned on the plane 11', resulting in defective contact overa large area between the mask plate 7 and the resist film 6. On theglass disc substrate of the shape shown in FIGS. 14 (b) or 14 (c), incontrast, the rise 8 of the resist film 6 is lower than the resist filmsurface plane 11 so that the mask plate 7 can be positioned on theresist film surface plane 11. Consequently, sufficient contact can beobtained between the mask plate 7 and the resist film 6 over the area 16with the guide grooves formed thereon.

As appreciated from the above, according to the present invention, theguide grooves formed in the optical memory element are entirely ofproper shape, thus reducing noise in reproduced signals.

While only certain embodiments of the present invention have beendescribed, it will be apparent to those skilled in the art that variouschanges and modifications may be made therein without departing from thespirit and scope of the present invention as claimed.

What is claimed is:
 1. A method for manufacturing an optical memoryelement having guide tracks and address tracks for storing information,which comprises the steps of:applying a photo resist film onto asubstrate which includes a center hole to form a coated substrate;preparing a photo mask plate by,forming a mask pattern on said maskplate, said mask pattern comprising a guide track pattern and addresstrack pattern, and forming a position reference marker on said maskplate with reference to said mask pattern for positioning said maskplate; placing said mask plate on said coated substrate so as toposition said position reference marker with reference to said centerhole of said coated substrate; transcribing, from said mask plate, alatent image of said mask pattern onto said photo resist film;developing said latent image on said photo resist film to form adeveloped photo resist film; and etching said substrate so as to formguide tracks and address tracks in said substrate in conformance withsaid mask pattern, whereby alignment between said center hole of saidsubstrate and said tracks is improved.
 2. The method of claim 1, whereinsaid substrate is disc shaped and wherein the shape of said positionreference marker corresponds to the shape of said center hole in saidsubstrate.
 3. The method of claim 1, wherein said mask plate includes aperipheral mask portion and wherein a part of said peripheral maskportion is removed before said mask plate is placed on said photo resistfilm so as to receive any risen portion of said photo resist film thatis of improper thickness whereby contact between said mask plate andsaid substrate is improved.
 4. The method of claim 1, wherein saidsubstrate includes a peripheral substrate portion and wherein saidperipheral substrate portion is removed before said photo resist film isapplied to said substrate so as to receive any risen portion of saidphoto resist film that is of improper thickness, whereby contact betweensaid mask plate and said substrate is improved.
 5. The method of claim2, wherein said mask plate is prepared by forming a metallic mask filmon said mask plate, and etching said mask film to form said mask patternand said position reference marker means on said mask plate.
 6. Themethod of claim 5, wherein said substrate comprises glass, said maskplate comprises quartz glass, and said metallic mask film comprises Cr,CrO, Ni, Ti, or Ta.
 7. The method of claim 6, wherein said photo resistfilm is applied on said substrate with a spin coating method, said maskplate is placed on said photo resist film in an airtight manner, saidmask pattern is transcribed onto said photo resist film by irradiatingsaid mask plate with ultraviolet rays, and said substrate is etched bywet etching or dry etching in a CF₄ or CHF₃ gas atmosphere.
 8. Themethod of claim 1, further comprising removing said developed photoresist film from said substrate and forming a recording medium layer onsaid substrate.
 9. The method of claim 7, wherein said photo resist filmhas a thickness of from about 100 nm to 500 nm.
 10. The method of claim5, wherein said position reference marker means comprises a mark of asimilar size as said center hole etched on said metallic mask film. 11.The method of claim 5, wherein said position reference marker comprisesa plurality of marks of various sizes compared to said center holeetched on said metallic mask film.
 12. The method of claim 3, whereinsaid peripheral mask portion is removed to form a mask groovecorresponding to the outer peripheral edge of said photo resist film.13. The method of claim 12, wherein said mask groove is formed to besquare-shaped, tapered, or curved.
 14. The method of claim 12, whereinsaid mask groove is formed with a depth of from about 0.2 to 0.5 mm. 15.The method of claim 4, wherein said peripheral substrate portion isremoved by grinding said substrate to form an outer inclined substrateedge which declines from the tracks plane of the portion of saidsubstrate adjacent to where said tracks are to be etched to below saidtracks plane at the outer edge of said substrate.
 16. The method ofclaim 15, wherein said outer edge of said substrate comprises achamfered edge.